The pathogenesis of diabetes mellitus is poorly understood. However, there is experimental evidence suggesting a similar underlying mechanism for the attenuated insulin secretory response observed in diabetes and in the fetal pancreas. The proposed research will examine the storage and secretion of insulin in beta cells during differentiation of the fetal pancreas. This research will allow characterization of the dynamics of insulin secretion during development of the fetal pancreas of lizards and rats. The proposed mammalian model is well suited since fetal beta cells of the rat, like human fetal beta cells have a reduced sensitivity to glucose. The saurian splenic pancreas has the advantage of being a model in which a high proportion of endocrine cells occurs. Beta cells of adult lizards (Anolis carolinensis) exhibit an attenuated insulin secretory response to glucose, and thus provide the opportunity for exploring the evolution of hyperglycemia during maturation of the endocrine pancreas. Differentiation of fetal, saurian and rat beta cells will be studied during normal development and during development in continuous perifusion-tissue culture. Functional development will be ascertained by following the accumulation of insulin within the fetal pancreas and by determining the dynamics of insulin release from the beta cell. Insulin levels will be quantified by means of radioimmunoassay. Structural differentiation of fetal endocrine pancreas will be followed by light and electron microscopy. Saurian embryos are collected from sacrificed females prior to oviposition and subsequently from incubated eggs. Rat embryos of 15-22 days gestation are obtained by cesarean section. The kinetics of insulin release in response to glucose and other insulin secretagogues will be determined by: 1) short-term perifusion of normal fetal pancreas, and 2) continuous perifusion-tissue culture of explants of fetal pancreas.